Study helps unravel the secret life of clouds

Air pollution can have a significant impact on the development of thunderclouds, causing cloud remnants to persist longer. bberwyn photo.

FRISCO — Air pollution can have a significant effect on the development of thunderhead clouds, causing the cloud remnants to persist high in the atmosphere long after thunderstorms dissipate. This, in turn, can affect daily temperature ranges, as the lingering clouds partially cool the Earth during the day with their shadows, but trap heat to keep nighttime temperatures warmer.

The new study, from the Department of Energy’s Pacific Northwest National Laboratory, helps answer long-running questions about how airborne pollutants affect climate warming. The findings will help provide a gauge for the accuracy of weather and climate models.

“This study reconciles what we see in real life to what computer models show us,” said atmospheric scientist Jiwen Fan. “Observations consistently show taller and bigger anvil-shaped clouds in storm systems with pollution, but the models don’t always show stronger convection. Now we know why.”

Researchers had thought that pollution causes larger and longer-lasting storm clouds by making thunderheads draftier through a process known as convection. But Fan and her colleagues show that pollution instead makes clouds linger by decreasing the size and increasing the lifespan of cloud and ice particles.

The study results may help fill in a few blank spots in climate models, which often replace storm clouds with simple equations that don’t tell the whole story.

The study suggests that the aerosols, which act as cloud seeding agents, result in more, but smaller cloud droplets, which changes cloud dynamics. Conventional wisdom holds that smaller droplets start a chain reaction that leads to bigger, longer-lasting clouds: Instead of raining down, the lighter droplets carry their water higher, where they freeze. The freezing squeezes out the heat the droplets carry with them and causes the thunder cloud to become draftier. The stronger convection lifts more water droplets, building up the cloud.

But according to observational data, the larger and longer-lasting clouds don’t always lead to stronger convection, especially in polluted skies.

The researchers started with cloud data from three locations that differ in how polluted, humid and windy they typically are: the tropics in the western Pacific, southeastern China and the Great Plains in Oklahoma.

The team found that in all cases, pollution increased the size, thickness and duration of the anvil-shaped clouds. However, only two locations — the tropics and China — showed stronger convection. The opposite happened in Oklahoma — pollution made for weaker convection.

This inconsistency suggested that stronger convection isn’t the reason. Taking a closer look at the properties of water droplets and ice crystals within clouds, the team found that pollution resulted in smaller droplets and ice crystals, regardless of location.

In addition, the team found that in clean skies, the heavier ice particles fall faster out of the anvil-shaped clouds, causing the clouds to dissipate. However, the ice crystals in polluted skies were smaller and too light to fall out of the clouds, leading to the larger, longer-lasting clouds.